This invention relates to a cam operated indexing drive and, more particularly, to a type two index right angle or roller gear index cam.
A cam operated index drive resembles a gear box. There is a continuously rotating cam-equipped input shaft which drives an intermittent motion output shaft. The output shaft has a complement of regularly spaced cam followers which mesh with the cam. The cam itself has two distinct sequences. Turning the camshaft while the output shaft is in the motion segment will produce a movement of the output shaft. Turning to the camshaft while the output shaft is in the dwell segment will produce no movement of the output shaft. For a type one cam there is only one motion segment followed by one dwell segment. For type two cams there are two complete sequences of motion and dwell--or see, for example, co-owned U.S. Pat. No. 3,049,017.
Cam operated indexing drives have high mechanical efficiencies and zero backlash. These two features are due to the fact that these indexing drives use roller bearings as cam followers. The rolling elements reduce friction and produce the high efficiency. Backlash is also known as lost motion. Zero backlash is sometimes called a positive connection. The outer shell of the cam follower is compressed or preloaded to provide the zero backlash. The cam follower is structured like an automobile wheel. There is a stud or spindle which is fixed to the follower wheel. There are small rollers which act like the rim of the wheel. Finally the cam follower has an outer shell which acts like the tire. The compressed shell resembles a car tire--it is round in shape but flat at the surface where it contacts the cam.
There are two techniques used to preload the cam followers. One technique is to manufacture the space between the cam followers with a dimension slightly greater than the actual spacing. This is the technique used to preload the right angle style of index cam. The second technique is to mount the input or cam shaft on an eccentric cartridge which allows an adjustment of the center distance between the input and output shafts. By turning the eccentric cartridge the cam is forced or wedged against the cam followers. This is the technique used to preload the roller gear style of index cam. The followers in an indexing cam will enter and leave the cam surface. At the points where they enter and leave the preload is relieved or tapered in order to allow a gradual build-up or preload.
The current technique commonly used by index cam manufacturers for dwells in a type two right angle or roller gear cam involve the alternating use of first two and then three followers in the dwell segments. In the two follower dwell segment there are two followers located symmetrically about the centerline of the cam blank, i.e., the equator of the generally cylindrical body constituting the cam. For the second dwell there are three followers, one on the centerline of the cam blank and the other two located symmetrically about the centerline. Initially, the preload is carried by the first and second followers through the first dwell and first motion. The third follower enters the cam during the end of the first motion but remains unpreloaded or relieved. During the second motion the first follower leaves the cam and preload is then passed to the second and third followers.
For the roller gear cams the preload is produced by decreasing the center distance between the input and output shafts. This decrease produces a deflection of the cam follower shell. But a problem arises when the cam is turned to the three follower dwell. There the same decrease has produced a greater amount of cam follower shell deflection. This alternating scheme does not produce balanced or identical dwell forces. There is some hand fitting required in order to get the correct feel. For the right angle which is preloaded by increasing the thickness of the track between the followers the problem is different. The forces between the two follower dwell are balanced but the forces for the three follower dwell are not. The inaccuracy of the type two right angle units is increased as a result of this.
The inventive follower preload sequence is identical to the current. The difference lies in rotating the two follower dwell by 1/4 of the spacing between consecutive followers. This rotation produces a first dwell which is the mirror image of the second dwell. The differences between the two dwells are eliminated along with their associated problems and the cams are statically balanced.
More specifically, the track forming wall means is shifted so that the dwells are on opposite sides of the equatorial centerline and there is no portion of the wall means paralleling the second dwell.